The present invention relates generally to the field of take-off devices, and, more particularly, to a screen printing take-off device which grips printed stock and moves the stock away from the printing device and then, in an improved and unusual manner opens and releases the stock.
Paper stock take-off devices are well known in the paper printing trade which grab the paper stock, such as a poster, play bill, etc., pull it away from the printing device and then release the stock, letting it fall to a conveyor belt or other receptacle below the take-off point. Because such printing systems commonly operate at a very high rate, for example, up to approximately 1000 cycles per hour, precise timing and functioning of all elements of the equipment is critical. Unfortunately, with known take-off devices paper jams are a common problem.
Conventional take-off devices have one or more gripper heads, each having upper and lower jaws which "bite" together to grip and pull a piece of paper stock. To release the paper stock after it has been completely removed from the printing machine, one of the jaws simply opens up or down, while the other (usually the bottom jaw) remains stationary. Alternatively, both jaws pivot simultaneously but in opposite directions.
A problem occurs with known take-off devices because of the high rate of speed at which the system is operating. The paper being removed from the printer is being pulled at such a great velocity, that when the upper and lower jaws of the take-off device release from one another the paper does not, and indeed cannot, simply drop directly downwardly away from the jaws merely because the previously gripping and pulling jaws have released. Instead, it continues to travel under its own momentum toward the open jaws of the take-off mechanism.
Having opened, the lateral velocity of the jaws dramatically decreases to zero so that the jaws can reverse both direction of travel and velocity to return to the screen printer to remove the next set of printed stock. Because the lateral velocity of the jaws is decreasing while the just-released printed stock is continuing under its own inertia and momentum, the stock occasionally becomes jammed in the jaws.
As a result, the entire printing process must be stopped while the jam in the take-off device is cleared. Because such printing systems commonly operate at such high speeds, possibly printing multiple copies with each cycle, significant production volume is lost with each stock jam, decreasing productivity and increasing labor and material costs.
The new screen printing take-off device described and claimed herein is provided with a unique structure and operation which overcome the above problems by virtue of a gripping head which does not simply open its "mouth", but instead combines a sliding action which releases the printed stock with a pushing or "knock-down" action. The effect of this combined action is to mechanically force and direct the stock downwardly toward the conveyor below the take-off device.
Designed as described hereafter for use with flat-bed printers, the new take-off device makes printing operations much more efficient by virtually eliminating the above-described "paper-hang", stock jams which bring the operation to a temporary halt and damage individual stock pieces in the process. As a result of this improved paper releasing function, the path of travel of the gripper head carriage can be shorter than is usually necessary for removal of the gripper heads from the line of movement of the paper stock being taken off the printing line. Accordingly, the overall. length of the chassis upon which the carriage travels is also shorter, requiring less floor space, and of course, less material to make the chassis, improving operating and manufacturing costs, respectively.
Thus, it is among the several objects of the-present invention to provide an improved print stock take-off device for screen printing operations which has a relatively simple construction and thus is facile and inexpensive to manufacture and yet operates at high rates of speed without experiencing the usual paper jams.
It is further among the objects of the invention having the features enumerated above, that the new take-off device be constructed so that it can be easily operated with little or no additional training by a single individual already able to operate conventional take-off devices, and that it require a minimal amount of maintenance to remain in ideal operating condition.
It is also among the objects of the present invention that the overall length of the new take-off apparatus be shorter than would ordinarily be required.
It is further among the objects of the present invention, having the features enumerated, that the new take-off apparatus be adjustable with regard to gripper tension and positioning to accommodate variations in stock size and image location on the stock.
Accordingly, in furtherance of the above objects, the present invention is, briefly, a take-off device for gripping and releasing an article. The take-off device includes a gripper head capable of imparting a downward force to an article gripped thefeb. The gripper head has a lower jaw and an upper jaw. The lower jaw is capable of moving linearly with respect to the longitudinal axis of the take-off device and the upper jaw is capable of pivotal movement below the line of linear movement of the lower jaw after the lower jaw has moved linearly to release the article gripped by the gripper head, whereby a downward force to an article gripped and released by the take-off device may be imparted.
The present invention is also, briefly, a screen printing take-off device for removal of printed paper stock from a printing machine. The device includes a chassis supported above a floor and having parallel right and left sides, relative to a printing machine opposite end view of the device. A carriage is movably mounted upon the chassis so as to travel along a longitudinal axis thereof. The carriage has a plurality of support rods mounted rotatably thereon, transversely in relation to the path of travel of the carriage on the chassis. The device also includes structure for causing the carriage to move forward and backward on the chassis, away from and toward the printing machine from which the stock is to be removed. The device further includes at least one gripper head mounted on the rotatable support bars perpendicularly thereto in such a fashion as to cause gripping and releasing of stock gripped thereby upon rotation of the support bars. The at least one gripper head has an upper jaw and a lower jaw. The upper jaw is pivotally mounted on the gripper head and the lower jaw is slidably mounted on the gripper head in such manner that the lower jaw slides to release the stock from the gripper head and the upper jaw pivots, pushing the stock, to thereby mechanically force and direct the released stock to fall clear from the gripper head. Also included is structure for causing the lower jaw to slide, to thereby release the stock gripped by the gripper head, and structure for causing the upper jaw to pivot after the lower jaw has slid, to thereby forceably change the direction of travel of the stock.
Other objects will be in part apparent and in part pointed out hereinbelow.